What is function composition?

Swift version: 5.6

Paul Hudson @twostraws May 28th 2019

Function composition is the ability to combine small functions together to make bigger functions.

In normal circumstances, you’d give some input to function A and get back output, and do the same for function B. When those two functions as combined together, A’s output becomes B’s input, so you provide input to A and get the results back from B.

To demonstrate this, here’s a function that generates random numbers in a specific range:

func generateRandomNumber(max: Int) -> Int {
    return Int(arc4random_uniform(UInt32(max)))
}

And here’s a function that spells out any number it’s given:

func spell(number: Int) -> String {
    let formatter = NumberFormatter()
    formatter.numberStyle = .spellOut
    return formatter.string(from: number as NSNumber) ?? ""
}

We can define a new operator that lets us combine those two together, like this:

precedencegroup CompositionPrecedence {
    associativity: left
}

infix operator >>>: CompositionPrecedence

func >>> <T, U, V>(lhs: @escaping (T) -> U, rhs: @escaping (U) -> V) -> (T) -> V {
    return { rhs(lhs($0)) }
}

You can learn more about how that works in Pro Swift.

You can now combine your two smaller functions into bigger ones, like this:

let spellOutRandom = generateRandomNumber >>> spell

spellOutRandom() is designed to take the input from the first function (a number) and send back the output from the second function (a string), so we can use it like this: